Mail processing systems for printing postage indicia on envelopes and other forms of mail pieces have long been well known and have enjoyed considerable commercial success. There are many different types of mail processing systems, ranging from relatively small units that handle only one mail piece at a time, to large, multi-functional units that can process thousands of mail pieces per hour in a continuous stream operation. The larger mailing machines often include different modules that automate the processes of producing mail pieces, each of which performs a different task on the mail piece. The mail piece is conveyed downstream utilizing a transport mechanism, such as rollers or a belt, to each of the modules. Such modules could include, for example, a singulating module, i.e., separating a stack of mail pieces such that the mail pieces are conveyed one at a time along the transport path, a moistening/sealing module, i.e., wetting and closing the glued flap of an envelope, a weighing module, and a metering module, i.e., applying evidence of postage to the mail piece. The exact configuration of the mailing machine is, of course, particular to the needs of the user.
Typically, a control device, such as, for example, a microprocessor, performs user interface and controller functions for the mailing machine. Specifically, the control device provides all user interfaces, executes control of the mailing machine and print operations, calculates postage for debit based upon rate tables, provides the conduit for the Postal Security Device (PSD) to transfer postage indicia to the printer, operates with peripherals for accounting, printing and weighing, and conducts communications with a data center for postage funds refill, software download, rates download, and market-oriented data capture. The control device, in conjunction with an embedded PSD, constitutes the system meter that satisfies U.S. information-based indicia postage meter requirements and other international postal regulations regarding closed system meters. The United States Postal Service (USPS) initiated the Information-Based Indicia Program (IBIP) to enhance the security of postage metering by supporting new methods of applying postage to mail.
The USPS has published draft specifications for the IBIP that define the requirements for the indicium to be applied to mail produced by closed systems. An example of such an indicium is illustrated in FIG. 1. The indicium 10 consists of a two-dimensional (2D) barcode 12 and certain human-readable information 14. Some of the data included in the barcode 12 can include, for example, the PSD manufacturer identification, PSD model identification, PSD serial number, values for the ascending and descending registers of the PSD, postage amount, and date of mailing. In addition, a digital signature is required to be created by the PSD for each mail piece and placed in the digital signature field of the barcode. Verification of indicium is performed by the postal service scanning a mail piece to read the 2D barcode 12 and verifying the information contained therein, including the digital signature. If the verification is unsuccessful, indicating that the indicium may not be authentic, the mail piece may not be delivered.
Since verification of the indicium requires reading the 2D barcode 12 and verifying the information contained therein, it is critical that the 2D barcode 12 be printed with sufficient resolution and clarity such that the scanners/readers are able to properly read and interpret the data. The 2D barcode 12 is approximately 0.8 inches by 0.8 inches and formed by a 40×40 array of pixels, with each pixel being 0.020 inches by 0.020 inches. Each pixel will be either black or white. As used herein, a black pixel indicates a pixel in which printing is performed, regardless of the ink color, and a white pixel indicates a pixel in which no printing is performed. If the pixel is black, a plurality of dots (depending upon the dots per inch of the printer used to print the barcode 12) are printed in the pixel. If the pixel is to be white, no dots are printed in the pixel. The data for the 2D barcode 12 is thus encoded as a series of black/white pixels and can therefore be read and interpreted by the verification equipment. One of the factors that affects the readability of the 2D barcode 12 is the size of each of the pixels (20 mils by 20 mils). If the size of each of the pixels is consistent, there is less chance of a pixel being incorrectly read and misinterpreted as either black or white. It is therefore important to maintain a consistent size for each of the pixels in the barcode 12.
In recent years, ink-jet printing systems have been utilized in mail processing systems. Ink-jet printing systems, as used herein, includes any form of printing wherein print control signals control a print mechanism to eject ink droplets from a plurality of nozzles to produce a matrix of pixels, i.e. picture elements, to represent an image. An ink supply, typically in the form of a reservoir, supplies ink to the print mechanism. FIGS. 2A-2D illustrate a cross-sectional view of an ink droplet 20 being discharged from a nozzle 16 of an ink-jet printing system onto a medium 18 passing beneath the nozzle 16 that is moving in the direction indicated by the arrow A. As shown in FIG. 2A, when droplet 20 is discharged from nozzle 16, it leaves a tail 22. The ink then begins to form a main droplet 20a and satellites 20b, 20c, as shown in FIG. 2B. The ink then splits into a main droplet 20a and satellites 20b, 20c as shown in FIG. 2C. The main droplet 20a contacts the medium 18 at a first location as shown in FIG. 2C. Since the medium 18 is moving in the direction indicated by arrow A, the satellite 20b will contact the medium 18 at a point to the left of the main droplet 20a as illustrated in FIG. 2D, and the satellite 20c will contact the medium to a point further left of the satellite 20b. The distance between the main droplet 20a and satellites 20b, 20c on the medium is dependent upon several factors, including the speed at which the medium is moving. Many current high speed mail processing systems will transport the medium at 70 inches per second (ips) or greater. The higher the speed of the medium, the greater the distance between the main droplet 20a and the satellites 20b, 20c. 
The formation of the satellites 20b, 20c can negatively impact the readability of the barcode 12 by impacting the size of the pixels through what is known as print growth. Print growth refers to the size of adjacent pixels in the barcode 12. FIG. 3 illustrates three pixels 30, 32, 34 from the barcode 12 of FIG. 1. As noted above, one of the factors that affects the readability of the 2D barcode 12 is the size consistency of each of the pixels 30, 32, 34. Readability will be best if the width a of pixel 30, the width b of pixel 32, and the width c of pixel 34 are equal. Thus, if a=b=c, the print growth with respect to pixels 30, 32, 34 will be zero. However, if the print growth between adjacent pixels is not zero, the readability will decrease. As shown in FIG. 4, the readability of the barcode 12 is greatest when the print growth is zero, and the readability decreases as the print growth increases. If pixel 32 is a black pixel, a plurality of dots will be printed in the pixel 32. If the medium on which the pixels 30-34 are located is moving to the right, such that pixel 34 will be printed first, pixel 32 printed next and then pixel 30 printed last, the dots being printed near the left edge of pixel 32 will have satellites that contact the medium in pixel 30. The satellites will effectively increase the width b of the pixel 32 and decrease the width a of the pixel 30. Thus, b will be greater than a, which results in a print growth of greater than zero, which reduces the readability of the barcode 12. As the speed of movement of the medium onto which the image is being printed increases, the satellites from the dots in pixel 32 will extend further into the pixel 30, further increasing the print growth and thus decreasing the readability. There is, therefore, a limitation imposed upon the speed of the medium when using drop-on-demand ink-jet technology to ensure that readability of the image will not be affected. In a mail processing system, this limitation determines the maximum speed at which the mail pieces can be transported and printed upon, and therefore limits the maximum throughput of the mail processing system. It would be desirable to be able to utilize drop-on-demand ink-jet technology for printing applications that does not impose these limitations with respect to the speed of the medium being printed upon, thereby enabling high speed printing.
Thus, there exists a need for a method and system that enables ink-jet printing technology to be used for high speed printing by compensating for droplet satellites to maintain readability of the images formed during printing.